Generally, an optical device such as a projector (e.g. an ultra-short focus projector) has a lens module, which is fixed on a holder and sheltered by a case structure. During the process of installing the lens module on the holder or transporting or using the optical device, the position of the lens module may be slightly shifted because of the external force. Even if a position shift is very tiny, it is difficult to adaptively adjust the optical parameters (e.g. focal length or magnification) of the lens module by the optical adjusting mechanism.
For avoiding generation of any tiny position shift of the lens module, a fixed lens module is disclosed. Since the fixed lens module fails to be adjusted, the precision of the molds and associated components should comply with a stringent standard. In this situation, the material cost and the fabricating cost are increased, and the fabricating process becomes complicated. Moreover, since the flange-back distance of the fixed lens module is unchanged, the projecting performance of the optical device with such fixed lens module is usually unsatisfied in some circumstances. Since the volume of the lens module of the current ultra-short focus projector is very bulky, the use of a focus-adjusting ring to adjust the focal length will adversely affect the appearance of the optical device.
Recently, the focal length of the optical device can be adjusted by using a remotely-controlled motor to adjust the lens module. This method, however, still has some drawbacks. For example, since the components are not cost-effective, the fabricating cost of the overall optical device is increased. In addition, due to the restriction of mechanical inertial properties, it is difficult to accurately adjust the focal length by the remote controller.
Therefore, there is a need of providing an optical device and an optical adjusting mechanism so as to obviate the drawbacks encountered from the prior art.